fixed sign of utc-gps time offset

[willat343]

When setting the CPT7 time with the local time, we are currently subtracting 18 seconds. I.e. we set UTC_time = GPS_time - 18 when it should be + 18.

An issue arises in the post-processing of the raw CPT7 IMU data where an extra -18 seconds are offset (correctly I believe) at

grand_tour_box/box_utils/box_auto/python/box_auto/scripts/cpt7/export_raw_imu_bag.py

Line 39 in b792920

self.expected_utc_offset = -18.0 # UTC time = GPS System Time + UTC offset

leading to a 36 second offset w.r.t. the other IMUs.

Thus datasets recorded to this point are 36 seconds offset, and in the post-processing should be corrected the other way.

Newer datasets will not have this problem.

Also this magic 18 number MIGHT change next year :)

[willat343]

This PR fixed 36 out of my 37 second offset between the CPT7 IMU (from log) and all the other IMUs, so i still have an unknown 1 second offset. I checked and it should still be 18 seconds. I will try find where this is coming from

[willat343]

Worth mentioning i get big offset warnings when processing this data (note that no GNSS data was available, so we set the CPT7 time locally for this mission)

WARNING: ⚠ UTC offset of 408931.74482028524 if different from expected.
WARNING: ⚠ GPS system offset 227653.59993281827 is different from expected